flow: optimize flow comparison

This patch reorder the order of check in CMP_FLOW to avoid to
IP address computation at first.

On test box, the result is going from 2.30% of sample to less than
2.00% in perf top. This should be better on a network with lot of
IPv6.

PR build: https://buildbot.suricata-ids.org/builders/regit/builds/48